Create SECURITY.md

This pull creates a new file in the top-level dir describing the project's security policy

Author: g-oikonomou

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+We will always apply vulnerability fixes on branch `develop` and include them
+in the next release. Depending on the severity, we will consider hotfixing
+the most recent release too. Older releases will not be hotfixed.
+
+| Version | Supported          |
+| ------- | ------------------ |
+| branch `develop` | :white_check_mark: |
+| [latest release](https://github.com/contiki-ng/contiki-ng/releases/latest)   | :question:                |
+| older releases   | :x:                |
+
+## Reporting a Vulnerability
+
+To report a vulnerability privately please email [email protected]
+
+We will try to confirm or decline your report ASAP by email. For accepted
+vulnerability reports we will create an [advisory](https://github.com/contiki-ng/contiki-ng/security/advisories)
+and, if you wish, invite you to the discussion.